Environmental concerns have motivated the implementation of emission requirements for internal combustion engines and other combustion systems throughout much of the world. Catalytic converters have been used to eliminate many of the pollutants present in exhaust gas; however, a filter is often required to remove particulate matter, such as, for example, ash and soot. Wall-flow particulate filters, for example, are often used in engine after-treatment systems to remove particulates from the exhaust gas.
The ability to accurately measure or estimate the amount of particulate, such as, for example, soot, accumulated in a particulate filter is valuable as it helps to determine the regeneration schedule for the filter. The pressure drop behavior of a fluid (e.g., post-combustion gas) flowing through a particulate filter may be used, for example, to estimate the particulate load in the particulate filter. For example, as a particulate accumulates in the filter, the pressure drop across the filter (e.g., from an inlet end to an outlet end) increases due to the increased restriction of the fluid (e.g., gas) passing through the filter's permeable particulate (e.g., including soot particulate and ash particulate) and porous wall layers. The particulate load (e.g., soot load) in a particulate filter can be estimated, for example, assuming a simple relationship between this pressure drop, the flow rate of the fluid flowing through the particulate filter, and the soot load. Such pressure drop based systems and methods have been shown to accurately estimate soot load in a particulate filter even under very dynamic operating conditions. Under certain engine operating conditions, however, pressure drop based systems and methods can have some limitations, thereby providing somewhat less accurate soot load estimates. One period during which pressure drop based systems and methods may provide less accurate estimates includes during significant variation in the ratio of engine nitrogen oxide (NOx) emissions (e.g., engine-out NOx) to engine particulate matter (PM) emissions (e.g., engine-out PM).
Extremely high ratios of engine-out NOx to engine-out PM (NOx/PM) may be encountered, for example, when an engine is running in a mode that reduces soot mass in the filter through passive regeneration. Conversely, extremely low engine-out NOx/PM conditions may be encountered when an engine is experiencing exhaust gas recirculation (EGR) problems, causing increased soot emissions. Such changes may affect the relationship between pressure drop and soot load, resulting in less accurate soot load estimates, which generally assume a constant soot layer state with minimal passive regeneration of the filter (i.e., that the soot layer forming in each channel of the particulate filter remains consistent).
It may be desirable, therefore, to provide a pressure drop based approach for estimating particulate load that provides relatively high levels of estimation accuracy during all periods of engine operation, including, for example, during variable NOx/PM engine-out conditions. It also may be desirable to provide a pressure drop based approach for estimating particulate load that is relatively simple to implement, using the instrumentation and sensors already available as part of an engine's after-treatment system.